Ultrasonic flow measuring devices of the aforementioned kind are used often in process- and automation-technology. They enable contactless determination of volume- and/or mass-flow of a medium in a pipeline. Clamp-on flow measuring devices are described, for example, in EP 0 686 255 B1, U.S. Pat. Nos. 4,484,478 or 4,598,593. Additionally, clamp-on flow measuring devices working on the basis of the travel-time difference method are sold by the assignee under the mark PROSONIC FLOW.
In the travel-time difference method, the different travel times of ultrasonic measuring signals in the stream direction and counter to the stream direction of the medium are evaluated. From the travel-time difference of the ultrasonic measuring signals, flow velocity and, therewith, in the case of known diameter of the pipeline, the volume flow rate, respectively in the case of known or measured density of the medium, the mass flow, can be ascertained.
In the case of the Doppler principle, ultrasonic measuring signals of predetermined frequency are coupled into the flowing medium. The ultrasonic measuring signals reflected in the medium are evaluated. On the basis of a frequency shift arising between the in-coupled and the reflected ultrasonic measuring signals, likewise, the flow velocity of the medium, respectively the volume- and/or mass-flow can be ascertained.
Use of flow measuring devices working according to the Doppler principle is only possible, when present in the liquid medium are air bubbles or impurities, on which the ultrasonic measuring signals can be reflected. Consequently, use of such ultrasonic flow measuring devices is relatively strongly limited in comparison to ultrasonic flow measuring devices working e.g. according to the travel-time principle.
In the case of ultrasonic flow measuring devices working on the basis of travel-time difference, Doppler, or cross-correlation methods, the ultrasonic measuring signals are coupled at a predetermined angle into, respectively out of, the pipeline, respectively measuring tube, in which the flowing medium is located. In order to achieve an optimum impedance matching and, therewith, optimum in- and out-coupling, the ultrasonic measuring signals are coupled into, respectively out of, the pipeline, respectively measuring tube, via an interfacing shoe, for example, in the form of a coupling wedge. A main component of an ultrasonic sensor is at least one piezoelectric element, which produces and/or receives the ultrasonic measuring signals in a defined frequency range.
The ultrasonic measuring signals produced in the piezoelectric element are conducted, via the coupling wedge (generically, the interfacing shoe) and the pipe wall, into the fluid medium. The medium is, as already mentioned, either a liquid or a gas. Due to the fact that the velocity of sound depends relatively strongly on material or medium, a refraction of the ultrasonic measuring signals occurs at the boundary between two different materials or media. The particular angle of refraction is calculated on the basis of Snell's Law, i.e. the angle of refraction depends on the ratio of the propagation velocities of two bordering materials or media. A representative assembly known from the state of the art, involving a piezoelectric element 5 and a coupling wedge 6 on a pipeline 3 containing a medium 4, is shown in FIG. 1.
Especially problematic is the in- and out-coupling of the ultrasonic measuring signals, when the pipeline is made of metal and a gaseous medium is flowing in the pipeline. Since the velocity of sound in metal and gas usually differs by an order of magnitude, a large part of the ultrasonic measuring signal is back-reflected, both in the case of in-coupling and in the case of out-coupling. The back-reflected portion is so large, that it is not possible to obtain reliable flow measurements with a conventional ultrasonic sensor. If yet additional sources of error arise, which relate, for instance, to the installation and assembly, or to changes in environmental conditions, then a conventional ultrasonic flow measuring device is just not suited for such an application.